Q I have noticed that the latest cartridges all have sharper shoulder angles and minimum body taper. What are the main advantages of this modernisation? Also, most of these new cartridges should be classed as being “over-bore capacity” which we know results in very short barrel life. What case capacity would you consider ideal in different calibres with regard to obtaining the best balance of performance and barrel life?
– Oliver Standish
A New cartridges generally feature a sharper shoulder angle of from 25 to 35 degrees. Theoretically, the sharply angled shoulder tends to hold the powder within the case and provide a little extra time to achieve better combustion. And with light to medium weight bullets the abrupt shoulder retains the powder while pressures build up behind the moderate weight bullet. It is significant, therefore, that all of the better performers among the magnum cartridges all feature shoulders with 25-35 degree angles. Minimum body taper not only provides greater powder capacity, but has the advantage of allowing the cartridge to extract more easily. Many high intensity magnum cartridges now use cases with a taper of only 0.0075-inch per inch. Cartridges that are over-bore capacity inevitably run into the law of diminishing returns, which is why barrel life of so-called super- magnums such as the 7mm STW etc rarely exceeds 800 rounds. It has pretty well been established that a .30-calibre cartridge is most efficient with a maximum capacity of 70gn of powder; the .25 calibre between 50 and 55 grains; and the .224 family not more than 40 grains. Go over these limits and the ratio between load and performance usually shows a deterioration in case and barrel life. Cartridges like the .308 Norma Magnum, .257 Roberts and .22-250 represent the optimum ratio between the total volume of the case and that of the bore because they have a high expansion ratio which is most desirable. Low expansion as in over-bore cartridges is not, as they give reduced barrel and case life and show a marked degree of inflexibility in their loadings.